Field of the Invention
The present invention relates to thermally insulating extruded thermoplastic polymer foam and a method for preparing such foam.
Introduction
Extruded polystyrene (XPS) foam is useful as thermal insulation. XPS foam has long been used as thermal insulation in building structures and containers. With an ever increasing drive for energy efficiency, there is an ever increasing drive to increase the thermal insulating properties of insulating products such as XPS foam. Therefore, it is desirable to identify how to decrease thermal conductivity through XPS foam.
Incorporating infrared attenuating agents such as carbon black and graphite into polymeric foam has been one approach for decreasing thermal conductivity through polymer foam. See, for example, U.S. Pat. No. 7,919,538B2. Conceptually, the infrared attenuating agents are dispersed within the cell walls of foam and absorb infrared radiation that is trying to pass through the foam. However, it is desirable to achieve even greater reduction in thermal conductivity through polymer foam than is achieved merely by including infrared attenuating agent in polymer foam as reported in U.S. Pat. No. 7,919,538.
Introduction of cell size anisotropy has also been an approach to minimizing thermal conductivity through polymer foam. As the following references indicate, the general consensus is that stretching foam in its extrusion direction so as to shorten the cell dimension in the vertical direction relative to dimensions perpendicular to the vertical direction of the foam increases thermal insulating properties (decreases thermal conductivity properties) through the foam.
U.S. Pat. No. 6,841,581 (Hayashi) refers to a cell anisotropic ratio k and specifies that when k exceeds 1.1 the thermal insulating property of foam decreases, making it difficult to obtain a short-term (30 day after production) thermal conductivity of 0.028 Watts per meter*Kelvin (W/m*K). The cell anisotropic ratio is defined as follows:k=a/(a*b*c)1/3 where a is the average cell size in the vertical direction, b is the average cell size in the transverse (horizontal) direction and c is the average cell size in the longitudinal (extrusion) direction.
EP1511795B1 and US2007/0142487 both teach that reducing the cell anisotropic ratio of foam increases the thermal insulation (reduces thermal conductivity) properties of the foam. EP1511795B1 and US2007/0142487 teach increased insulation is obtained with foam having a value for x/z (which corresponds to c/a in the above notation) that is between 1.03 and 2.0. That corresponds to a “z/x” value, which is the same as “a/c” in terms of U.S. Pat. No. 6,841,581, that is less than one. These references encourage flattening cells in a foam's vertical direction to achieve lower thermal conductivity through the foam's thickness.
U.S. Pat. No. 6,315,932 further teaches increased thermally insulating properties are achieved in foam when the z/x ratio (a/c ratio in above notation) is one or less, thereby motivating flattening of cell dimensions in the vertical direction of foam to achieve enhanced thermally insulating properties.
EP561216B2 further directs flatting foam in its vertical direction to obtain increased thermal insulating properties.
It would advance the art of thermally insulating XPS foam to discover a new way to obtain an XPS foam that has a thermal conductivity, especially a long-term (25-year) thermal conductivity of 0.030 W/m*K or less, preferably 0.028 W/m*K or less, especially while also having a compressive strength of 200 kilopascals or more. Long-term thermal conductivity corresponds to thermal conductivity 25 years after manufacture as opposed to merely 30 days after manufacture as is the case with short-term thermal conductivity. Achieving a low “long-term” thermal conductivity is more challenging than achieving a low “short-term” thermal conductivity because thermal conductivity tends to increase over time as thermally insulating blowing agent gas in the cells permeates out from the foam.